The statements in this section merely provide background information related to the present teachings and may not constitute prior art.
Moveable components, such as control valves, have performance limitations due to hysteresis. In the case of a valve, the hysteresis is a limitation of the valve to return to a specific control output given an identical control input. For example, the ability of the valve to return to a specific control output, given an identical control input, can vary depending upon whether the control point is approached from below or approached from above. In other words, if a control input to the valve is a command to go 50% open, the valve may go to 48% open when approaching the control point, from say 40% open originally, and may go to 52% open when approaching the control point, from say 60% open originally, giving a hysteresis of ±2%.
The hysteresis, in the case of a valve, can be caused by several factors. The factors can include, by way of non-limiting example, mechanical slop in the valve linkages, mechanical slop in the feedback linkages, mechanical or electrical slop in the feedback sensor, mechanical backlash, and imposed software hysteresis bands or “dead band” for a given closeness of the desired control point compared to the feedback signal from the valve, etc., or some combination thereof, depending on the particulars of the valve and control methodology. The “dead band” can be considered to be a region wherein the valve is close enough to the actual/desired value that the control system will not call for any further changes to the valve position.
In some applications, depending on the loading conditions and other factors, the hysteresis can prevent proper control of the system, in particular when small changes in control are required to adjust about a setpoint to account for setpoint variations or to mitigate the effects of disturbances. The hysteresis can lead to either no output change for small control input change, or to excessive change for a small control input change. Both situations can lead to the output not matching the desired control point.
To reduce the hysteresis for a component, expensive precise control linkages and mechanisms can be utilized to reduce the mechanical slop. Additionally, better performing or more precise sensors can also be utilized to reduce the electrical slop in the feedback loop. The result, however, can be a control arrangement that is expensive and increases the overall costs of the system within which the component and the associated control system are utilized.